For cross-country mountain biking...

Is it absolutely nesessary to master the log riding or being able to ride on these "north shore" style ramps like the one here: http://www.mountainbikesapart.com/wp...bike_trail.jpg ?
Some of them, ESPECIALLY the thin ones are intimidating for me, so I am wondering whether this is suitable for cross-country biking or for more extreme mountain biking like downtill, freeride, ect...

I presume you mean the 'narrow ones' ...

You should ask any of your buddies what the expected speed is that you're supposed use when riding one of those courses ...

Hopefully, they'll tell you to ride at the speed that you are most comfortable with ... and, faster riders will just have to wait behind you as you negotiate some parts.

BTW. The section of the course in the pic is not extreme ... however, I've seen some sections that 'I' would definitely classify as extreme.